In connection with the technology regarding glass bottles with an outer thread at the mouth, a basic distinction must be made between two types of screw caps, namely, leak-proof screw caps for liquids on the one hand, which are generally known as so-called pilfer-proof closures or PP-closures for short, and on the other hand screw caps which are air-tight, also known in the trade as GD-closures. The first type of closure is used with so-called still beverages such as, for example, spirits. In this type of closure the seal is made between the upper end face of the screw cap, which normally contains the seal, and the upper edge of the mouth of the glass bottle. By means of the active thread, i.e. the outer thread of the glass bottle, together with the thread face of the screw cap created by screwing it down, sufficient pressure can be maintained. However, because there is no air-tight closure, the system in question must be classified only as leak-proof.
Therefore such closures are unsuited for carbonated beverages. As a rule, these are bottled under a pressure of several bar, e.g. 4 to 6 bar. They require air-tight closures in which an additional seal is attained by deep-drawing of the transition area between the upper end face and the lateral thread face of the screw cap onto the lateral edge of the mouth of the glass bottle. This system can be classified as extremely tight and pressure retaining. In view of the many carbonated beverages which are bottled in this manner and also because of its basic suitability for bottling fruit juices in accordance with this system, i.e. to seal them, the technology is used, mostly without change, for reasons of efficiency (to avoid additional outlays for tools and retrofitting) in connection with fruit juices.
In any case, air-tight screw caps are obviously used in considerable numbers. Thus it is possible that troubles of grave consequence can occur--even though only occasionally. Depending on the degree to which a glass bottle containing a carbonated beverage is emptied and on the action of the outside temperature, internal pressures can easily build up which considerably exceed the internal pressure at the time of bottling. This can result in the screw cap blowing off and the glass bottle "venting". The blowing off of the screw cap can be extremely violent, even like the firing of a projectile, and therefore is very dangerous. However, this must be considered as the lesser evil. It is not always the screw cap which in such cases represents the weakest point, sometimes the glass bottle itself cannot withstand such internal pressure. This frequently leads to an explosive destruction of the glass bottle which, if people are present, can result in extremely dangerous injuries. This has been repeatedly reported in the press.
But fruit juice bottles provided with air-tight screw caps also are a source of serious danger. Normally, fruit juices are bottled while hot, which at first results in a vacuum; however, an internal pressure can also build up with fruit juices as a result of fermentation. This is all the more noteworthy, because often glass bottles with lesser wall thicknesses are chosen for fruit juices. Thus, there is also a considerable danger of injuries in these cases. Different screw caps are known, particularly in connection with fruit juices, namely those with a circular stamped area in the center of the upper end face which permits the central part thus defined to either bend downwardly as an indicator of a vacuum or, if the internal pressure is unacceptably high, to break off along the stamped line (the predetermined breaking point). However, these methods do not apply if, under standard practices--as previously stated, for reasons of cost--, carbonated beverages and fruit juices are being treated in the same way, i.e. are bottled with the same automatic machinery without modification.